Recently, electromagnetic waves, which have Orbital Angular Momentum (OAM), are used in wireless communication in which signals are transmitted and received using the electromagnetic waves.
In the electromagnetic waves having the OAM, a plurality of modes (hereinafter, described as OAM propagation modes) exist according to difference in phase variations with respect to an azimuth angle. The electromagnetic waves having different OAM propagation modes are orthogonal to each other.
For example, PTL 1 discloses a technology in which a plurality of transmission parabola antennas transmit electromagnetic waves, which respectively have different OAM propagation modes, and a plurality of reception parabola antennas, which form pairs with the plurality of transmission parabola antennas, receive the electromagnetic waves which are transmitted from the relevant transmission parabola antennas. According to the technology, it is possible to transmit and receive the electromagnetic waves, which have one OAM propagation mode, between a pair of the transmission parabola antenna and the reception parabola antenna, and thus it is possible to realize high communication capacity by multiplexing signals over a plurality of orthogonal channels.
In addition, PTL 2 and PTL 3 disclose technologies for transmitting the electromagnetic waves having different OAM propagation modes using an array antenna disposed in a circle. According to the technologies, it is possible to transmit and receive the electromagnetic waves having the plurality of OAM propagation modes, and thus it is possible to realize the high communication capacity by multiplexing the signals over the plurality of orthogonal channels.